1. Field of the Invention
The present invention relates to a light emitting device and to a display device. In addition, the present invention relates to electronic equipment in which the light emitting device or the display device is mounted. The term light emitting device as used in this specification indicates devices that utilize light emitted from a light emitting element. Examples of the light emitting elements include organic light emitting diode (OLED) elements, inorganic material light emitting diode elements, field emission light emitting elements (FED elements) and the like. The term display device as used in this specification indicates devices in which a plurality of pixels are arranged in a matrix shape, and image information is visually transmitted, namely displays.
2. Description of the Related Art
The importance of display devices that perform display of images and pictures has continued to increase in recent years. Due to their advantages such as high image quality, thin size, and light weight, liquid crystal display devices that perform display of an image by using liquid crystal elements are widely utilized in various types of display devices, such as portable telephones and personal computers.
On the other hand, the development of display devices and light emitting devices that use light emitting elements is also advancing. Elements that use many different types of materials over a wide-ranging area, such as organic materials, inorganic materials, thin film materials, bulk materials, and dispersed materials exist as light emitting elements.
Organic light emitting diodes (OLEDs) are typical light emitting elements currently seen as promising for all types of display devices. OLED display devices that use OLED elements as light emitting elements are thinner and lighter than existing liquid crystal display devices, and in addition, have characteristics such as high response speed suitable for dynamic image display, a wide angle of view, and low voltage drive. A wide variety of applications are therefore anticipated, from portable telephones and portable information terminals (PDAs) to televisions, monitors, and the like. OLED display devices are under the spotlight as next generation displays.
In particular, active matrix (AM) OLED display devices are capable of high resolution (large number of pixels), high definition (fine pitch), and a large screen display, all of which are difficult for passive matrix (PM) type displays. In addition, AM-OLED display devices have high reliability at lower electric power consumption operation than that of passive matrix OLEDs, and there are very strong expectations that they will be put into practical use.
OLED elements are structured by an anode, a cathode, and an organic compound containing layer sandwiched between the anode and the cathode. Normally the brightness of light emitted from the OLED element is roughly proportional to the amount of electric current flowing in the OLED element. A driver transistor that controls the light emission brightness of a pixel OLED element is inserted in series with the OLED element in AM-OLED display device pixels.
Voltage input methods and current input methods exist as driving methods for displaying images in AM-OLED display devices. The voltage input method is a method in which a voltage value data video signal is input to the pixels as an input video signal. On the other hand, the current input method is a method in which a current value video signal is input to the pixels as an input video signal.
The video signal voltage is normally applied directly to a gate electrode of a pixel driver transistor in the voltage input method. If there is dispersion, not uniformity, in the electrical characteristics of the driver transistors across each of the pixels when the OLED elements emit light at a constant current, then dispersion will develop in the OLED element driver current of each of the pixels. Dispersion in the OLED element driver current becomes dispersion in the brightness of light emitted from the OLED elements. Dispersion in the brightness of light emitted by the OLED elements reduces the quality of the displayed image as a sandstorm state or carpet-like pattern unevenness is seen over an entire screen. Stripe shape unevenness is also found, depending upon the manufacturing process.
In particular, a relatively large electric current is necessary in order to obtain a sufficiently high brightness when OLED elements presently capable of being used, which have low light emission efficiency, are applied as a light emitting device. As a result, it is difficult to use amorphous silicon thin film transistors (TFTs), which have low current capacity, as the driver transistors. Polycrystalline silicon (polysilicon) TFTs are therefore used as the driver transistors. However, there is a problem with polysilicon in that dispersions in the TFT electrical characteristics are likely to develop due to causes such as faults in the crystal grain boundaries.
The current input method can be used as one effective means in order to prevent dispersion in the OLED element driver current that occurs in this type of voltage input method. A video signal data current value is normally stored with the current input method, and an electric current identical to, or several times as large as, the value of the stored electric current (positive real number multiples, including those less than 1) is supplied as the OLED element driver current.
A typical known example of a pixel circuit of a current input method AM-OLED display device is shown in FIG. 10A (refer to Non-Patent Document 1). Reference numeral 516 denotes an OLED element. This pixel circuit uses a current mirror circuit. Video signal data current values can be accurately stored as long as two transistors structuring the current mirror have identical electrical characteristics. Even if there is dispersion in the electrical characteristics of the driver transistors of different pixels, dispersion in the brightness of light emitted by the OLED elements can be prevented as long as the two transistors within the same pixel each have identical electrical characteristics.
Another typical known example of a pixel circuit of a current input method AM-OLED display device is shown in FIG. 10B (refer to Non-Patent Document 2). Reference numeral 611 denotes an OLED element. This pixel circuit has a short circuit between a drain electrode, and a gate electrode, of a driver transistor itself when a voltage corresponding to a video signal is written into the gate electrode of the driver transistor. A video signal data current is made to flow in this state, and the gate electrode is then electrically insulated. By doing so, an electric current having a value identical to the data current during write-in is supplied to the OLED element by the driver transistors, provided that the driver transistors are operated in the saturated region. Dispersion in the brightness of light emitted by the OLED elements can therefore be prevented, even if dispersion exists in the electrical characteristics of the driver transistors of each pixel.
[Non-Patent Document 1] Yumoto, A., et al., Proc. Asia Display/IDW ""01, pp. 1395-1398 (2001).
[Not-Patent Document 2] Hunter, I. M., et al., Proc. AM-LCD 2000, pp. 249-252 (2000).
The data current value should be able to be accurately stored with FIGS. 10A and 10B, as discussed above, but there are serious problems as stated below.
First, a problem with the pixel circuit of FIG. 10A is that there is a precondiction in which the two transistors 512 and 513 that structure the current mirror must have identical electrical characteristics. Provided that it is considered during design, it is possible to manufacture both transistors adjacently on a substrate, and dispersion can be reduced to a certain extent. However, dispersions in the electrical characteristics of TFTs, such as threshold voltage and field effect mobility, that exceed a permissible limit normally remain in present-day polysilicon due to causes such as faults in the crystal grain boundaries.
Specifically, it becomes necessary to keep brightness within a range on the order of 1%, for example, if a 64 gray scale image is displayed. However, storing the data current values at a precision of 1% with the pixel circuit of FIG. 10A is difficult to achieve with the polysilicon normally in use at present. In other words, a sufficiently uniform, high quality display image over an entire screen, without irregularities, cannot be obtained by only using the pixel circuit of FIG. 10A.
Next, the fact that the video signal data current written into the pixel has the identical value to the OLED element driver current when the OLED element emits light is a problem with the pixel circuit of FIG. 10B. The fact that both electric currents must have identical values is a very severe restriction in practice when manufacturing an AM-OLED display device.
Specifically, a large amount of parasitic capacitance and parasitic resistance exists in signal lines and the like in an actual AM-OLED display device. As a result, it often becomes necessary to take steps to make the video signal data current larger than the OLED element driver current. In particular, it becomes extremely difficult to write in the video signal data current of dark portions for cases in which the video signal data current is made into an analog value for gray scale expression.
The present invention has been made in view of the aforementioned problem points. First, an object of the present invention is to provide an AM-OLED display device in which the ratio between a video signal data current written into a pixel, and an OLED element driver current during OLED element light emission, is not fixed to a value of xe2x80x9c1xe2x80x9d, differing from the pixel circuit of FIG. 10B. Next, the present invention is premised on the fact that it is possible for dispersion in electric characteristics to remain to a certain extent, even between transistors placed adjacently within the same pixel, differing from the pixel circuit of FIG. 10A. Therefore, another object of the present invention is to provide an AM-OLED display device in which dispersion in the OLED element driver currents is sufficiently inhibited compared to pixel circuits that use a current mirror like that of FIG. 10A.
Note that the constitution of the present invention can be effectively utilized when using current driven elements in display devices and light emitting devices that use elements other than OLED elements.
In order to solve the aforementioned objectives, the present invention is characterized in that driver elements disposed in each pixel of an AM display device or a light emitting device are structured by a plurality of transistors, the plurality of transistors are placed in a parallel connection state when a data current is written into the pixel, and the plurality of transistors are placed in a series connection state when a light emitting element emits light.
Note that the constitution of the present invention can be utilized when using current driven elements in display devices and light emitting devices that use elements other than OLED elements.
An outline of the pixel structure of this type of display device or light emitting device of the present invention is explained using FIGS. 1A and 1B. FIG. 1A shows a pixel 11 disposed in a j-th row and an i-th column in a pixel portion having a plurality of pixels. The pixel 11 has a signal line (Si), a power source line (Vi), a first scanning line (Gaj), a first switch 12 having a switching function, a second switch 13 having a switching function, a third switch 14 having a switching function, a driver element 15, a capacitor element 16, and a light emitting element 17. Note that it is not always necessary to form the capacitor element 16 for cases such as those where the parasitic capacitance of a node at which the capacitor element 16 is disposed is large.
An OLED element is typically applied as the light emitting element, and therefore a diode reference symbol may also be used in this specification as a reference symbol that expresses the light emitting element. However, diode characteristics are not necessary in the light emitting element, and the present invention is not limited to light emitting elements that possess diode characteristics. In addition, the light emitting elements in this specification may be current driven display elements, and it is not necessary that the elements have a display function due to emitted light. For example, light shutters such as liquid crystals that can be controlled by electric current values, not voltage values, are also included in the category of light emitting elements in this specification.
One semiconductor element, or a plurality of semiconductor elements, having a switching function, such as a transistor can be used in the first switch 12, the second switch 13, and the third switch 14. A plurality of semiconductor elements such as transistors can also be used similarly in the driver element 15. On and off states for the first switch 12 and the second switch 13 are determined by signals imparted from the first scanning line (Gaj). It is sufficient that the first switch 12 and the second switch 13 function as switching elements, and therefore no particular limitations are placed on the conductivity type of the semiconductor elements used.
Note that the first switch 12 located between the signal line (Si) and the driver element 15, and plays a role in controlling signal write-in to the pixel 11. Further, the second switch 13 is located between the power source line (Vi) and the driver element 15, and controls the supply of electric current form the power source line to the pixel 11.
A case of additionally disposing a fourth switch 18 and a second scanning line (Gbj) in the pixel 11 of FIG. 1A is shown in FIG. 1B. One semiconductor element, or a plurality of semiconductor elements, having a switching function, such as transistors, can be used in the fourth switch 18. On and off states for the fourth switch 18 are determined by signals imparted from the second scanning line (Gbj). It is sufficient that the first switch 12 and the second switch 13 function as switching elements, and therefore no particular limitations are placed on the conductivity type of the semiconductor elements used.
Note that the fourth switch 18 plays a role as an initialization element for the pixel 11. Electric charge stored in the capacitor element 16 is released if the fourth switch 18 turns on, the driver element 15 turns off, and in addition, light emission by the light emitting element 17 stops.
The present invention is characterized in that the driver element 15 is structured by a plurality of transistors, and the connection between the plurality of transistors is switched to a parallel connection for cases in which a video signal data current is written into the pixel 11, or to a serial connection for cases in which electric current flows in the light emitting element 17, which thus emits light. On and off control of the first switch 12 and the second switch 13 by signals from the scanning line (Gaj) in FIGS. 1A and 1B becomes a means for switching the plurality of transistors in the driver element 15 between a parallel connection state and a series connection state.
Examples of the pixel 11 for a case of structuring the driver element 15 by using four transistors 20a, 20b, 20c, and 20d are shown in FIGS. 1C and 1D. Explanations of current pathways in the pixel 11 are provided below.
FIG. 1C shows a case of writing a data current into the pixel 11, and FIG. 1D shows a case of the light emitting element emitting light. Note that elements other than the first switch 12, the second switch 13, the driver element 15, the light emitting element 17, the signal line (Si), and the power source line (Vi) are not shown in FIGS. 1C and 1D.
A case in which a data current is written into the pixel 11 is explained first. The first switch 12 and the second switch 13 turn on due to a signal imparted from the first scanning line (Gaj) in FIG. 1C. Each transistor in the driver element 15 is thus placed in a diode connected state, and all of the transistors are mutually connected in a parallel connection state. A current pathway exists from the power source line (Vi), through the second switch 13, the driver element 15, and the first switch 12, to the signal line (Si). A current value IW at this point is the data current value of the video signal, and is a predetermined current value output to the signal line (Si) by a signal line driver circuit.
A case in which the light emitting element 17 emits light is explained next. The first switch 12 and the second switch 13 are turned off by a signal imparted from the first scanning line (Gaj) in FIG. 1D. Each of the transistors in the driver element 15 are thus mutually connected in a series connection state. A current pathway exists from the power source line (Vi), through the transistors 20a, 20b, 20c, and 20d, to the light emitting element 17. The brightness of light emitted by the light emitting element 17 is determined by a current value IE at this point.
As discussed above, the transistors 20a to 20d that structure the driver element 15 are used in parallel with the present invention during write-in of the data current to the pixel (see FIG. 1C). In addition, the transistors 20a to 20d that structure the driver element 15 are used in series when electric current flows in the light emitting element 17 of the pixel 11, that is when the light emitting element is driven (see FIG. 1D). The current value IW during write-in therefore becomes 16 times (42 times) the current value IE during light emitting element drive, if it is assumed that the electrical characteristics of the transistors 20a to 20d are identical. In general, if the number of transistors structuring the driver element 15 is considered to be n, then a relationship shown by Eq. 1 is established between the current value IW during video signal write-in and the current value IE during light emitting element drive, under the condition that all of the transistors have identical electrical characteristics.
IW=n2xc3x97IExe2x80x83xe2x80x83(1) 
Here, n is preferably between 3 and 5. Note that, in order to strictly establish Eq. 1, there is a condition that all of the transistors structuring the driver element 15 must possess identical electrical characteristics. However, it is possible in practice to treat Eq. 1 as if approximately established, even for cases involving a slight amount of mutual dispersion in the electrical characteristics of the transistors.
Thus, the present invention is characterized in that the driver element 15 is structured by the plurality of transistors, and the current value IW during write-in, and the current value IE during light emitting element drive, can therefore be arbitrarily set by switching the connection between the plurality of transistors between parallel and serial for cases of writing a video signal current into the pixel 11 and for cases of the light emitting element emitting light.
Further, the present invention is also characterized in that the influence of slight, mutual differences in the electrical characteristics of each of the transistors structuring the driver element 15 can be greatly reduced from being reflected in the light emitting element drive current IE. A specific example of this is taken up and explained in an embodiment mode.
Even with a pixel circuit using a current mirror like that of FIG. 10A, there is a problem in that identical electrical characteristics are required for the two transistors within the pixel. However, even the transistors within the same pixel are already presupposed to have slightly different electrical characteristics in the present invention. That is, the present invention is superior compared to pixel circuits that use current input method current mirrors in that the present invention has tolerance for dispersions in the characteristics of the transistors. As a result, it becomes possible to make the light emitting element driver current IE uniform to a level at which it can be put into practical use, even if dispersions in the electrical characteristics of polysilicon TFTs, caused by defects in crystal grain boundaries and the like, exist.
The display device and the light emitting device of the present invention are display devices provided with a plurality of pixels. The pixels each have a driver element provided with a light emitting element and a plurality of transistors. The display device and the light emitting device of the present invention are characterized by including a means capable of making, at minimum, a state in which the plurality of transistors in the driver element are connected in parallel, and a state in which the plurality of transistors in the driver element are connected in series. The term light emitting device as used in this specification indicates devices that utilize light emitted form a light emitting element. Examples of light emitting elements include organic light emitting diode (OLED) elements, inorganic material light emitting diode elements, and field emission light emitting elements (FED elements). The term display device as used in this specification indicates devices in which a plurality of pixels are arranged in a matrix shape, and image information is transferred visually, namely displays.
An outline of a pixel structure of the display device and the light emitting device of the present invention that differs from that of FIGS. 1A and 1B is explained here using FIGS. 11A and 11B. The pixel 11 disposed in the j-th row and the i-th column in the pixel portion having a plurality of pixels is shown in FIG. 11A. The pixel 11 of FIG. 11A is provided with a signal line (Si), a power source line (Vi), a first scanning line (Gaj), a second scanning line (Gbj), a third scanning line (Gcj), a fourth scanning line (Gdj), a first switch 312, a second switch 313, a third switch 314, a fourth switch 318, a driver element 315, a capacitor element 316, a light emitting element 317, and an opposing electrode 319, for example. However, even if the structure with the first switch, the second switch, the third switch, the fourth switch, the first scanning line (Gaj), the second scanning line (Gbj), the third scanning line (Gcj), the fourth scanning line (Gdj), and the like is changed slightly, in practice the same device can be obtained. One example of such is FIG. 11B. The fourth switch is removed, and the third scanning line is unified with the second scanning line in FIG. 11B. This is also identical in practice to FIG. 11A, and in the absence of any specific limitations, is taken as being included in FIG. 11A. Cases of adding components such as initialization elements are also similarly treated.
Note that the capacitor element 316 does not always have to be expressly formed in FIGS. 11A and 11B for cases in which the parasitic capacitance of a node at which the capacitor element 316 is disposed is large, and the like.
A single semiconductor element, or a plurality of semiconductor elements, having a switching function such as transistors, can be used in the first switch 312, the second switch 313, the third switch 314, and the fourth switch 318. A plurality of semiconductor elements such as transistors can also be similarly used in the driver element 315. There are no particular limitations placed on the conductivity type (n-channel, p-channel) of the semiconductor elements used in the first switch 312, the second switch 313, the third switch 314, the fourth switch 318, and the driver element 315. This is mostly because n-channel and p-channel types can both be used, and there are cases in which a specified conductivity type is more preferable than another conductivity type for specific applied examples.
A signal imparted from the first scanning line (Gaj) determines whether the first switch 312 is on or off. Similarly, a signal form the second scanning line (Gbj) determines whether the second switch 313 is on or off, a signal from the third scanning line (Gcj) determines whether the third switch 314 is on or off, and a signal from the fourth scanning line (Gdj) determines whether the fourth switch 318 is on or off. It is of course not necessary for all of the scanning lines, the first scanning line (Gaj), the second scanning line (Gbj), the third scanning line (Gcj), and the fourth scanning line (Gdj), to exist, and a certain scanning line can also be combined with other scanning lines, as is made clear by FIG. 11B.
The first switch 312 is disposed between the signal line (Si) and the driver element 315 in FIG. 1A, and serves a role for controlling signal write-in to the pixel 11. Further, the second switch 313 and the fourth switch 318 are disposed between the power source line (Vi) and the driver element 315, and perform on and off control of the supply of electric current form the power source line (Vi) to the pixel 11. The third switch 314 is disposed between the driver element 315 and the light emitting element 317, and performs on and off control of the supply of electric current form the driver element 315 to the light emitting element 317.
In the present invention, the driver element 315 is structured by the plurality of transistors, and the plurality of transistors are connected in parallel when a video signal data current is written into the pixel 11. The plurality of transistors are connected in series when electric current flows in the light emitting element 317, and light is emitted. It becomes possible to place the plurality of transistors in the driver element 315 in a parallel connection state, and also in a series connection state, by controlling the on and off states of the first switch, the second switch, the third switch, and the fourth switch using the signals from the scanning lines (Gaj, Gbj, Gcj, and Gdj) in FIG. 11A.
The pixel 11 is shown in FIGS. 11C and 11D here as an example of a case in which the driver element 315 is structured by four transistors 320a, 320b, 320c, and 320d. Electric current pathways in the pixel 11 are explained below.
FIG. 11C shows a case of writing a data current into the pixel 11, and FIG. 11D shows a case of the light emitting element emitting light. With FIG. 11C, the four transistors 320a, 320b, 320c, and 320d are in a parallel connection state, while the four transistors 320a, 320b, 320c, and 320d are in a series connection state in FIG. 11D. Note that element and wirings other than the first switch 312, the second switch 313, the driver element 315, the light emitting element 317, the source signal line (Si), and the power source line (Vi) are, omitted from being shown in FIGS. 11C and 11D.
A case of writing a data current into the pixel 11 is explained first. The first switch 312 and the second switch 313 are turned on in FIG. 11C by signals imparted from the first scanning line (Gaj) and the second scanning line (Gbj), respectively. Each of the transistors in the driver element 315 is thus placed into a diode connected state, and the transistors are thus mutually placed in a parallel connection state. The third switch 314 and the fourth switch 318 turn off by signals input from the third scanning line (Gcj) and the fourth scanning line (Gdj), respectively. A current pathway exists from the power source line (Vi), through the second switch 313, the driver element 315, and the first switch 312, to the signal line (Si) when the power source line (Vi) has a high electric potential. The opposite is naturally true if the power source line (Vi) has a low electric potential. The current value IW is the value of the video signal data current at this point, and is a predetermined current value output to the signal line (Si) from a signal line driver circuit.
A case of the light emitting element 317 being made to emit light is explained next. The first switch 312 and the second switch 313 are turned off by signals imparted form the first scanning line (Gaj) and the second scanning line (Gbj), respectively, in FIG. 11D. The transistors in the driver element 315 are thus mutually placed in a series connection state. The third switch 314 and the fourth switch 318 turn off due to signals imparted form the third scanning line (Gcj) and the fourth scanning line (Gdj), respectively. A current pathway exists from the power source line (Vi), through the transistors 310a, 320b, 320c, and 320d, and to the light emitting element 317 when the power source line (Vi) has a high electric potential. The opposite is naturally true if the power source line (Vi) has a low electric potential. The current value IE determines the brightness of light emitted by the light emitting element 317 at this point.
The transistors 320a, 320b, 320c, and 320d that structure the driver element 315 are used parallelly when writing a data current into the pixel in the present invention (see FIG. 11C). On the other hand, the transistors 320a, 320b, 320c, and 320d that structure the driver element 315 are used serially when electric current flows in the light emitting element 317 of the pixel 11, that is when the light emitting element is driven (see FIG. 11D). The current value IW during write-in therefore becomes 16 (42) times the current value IE when the light emitting element is driven, provided that the electrical characteristics of the transistors 320a, 320b, 320c, and 320d are presumed to be identical. In general, if the number of transistors structuring the driver element 15 is considered to be n, then the relationship shown by Eq. 1 is established between the current value IW during video signal write-in and the current value IE during light emitting element drive, under the condition that all of the transistors have identical electrical characteristics.